1. Field of the Invention
This invention relates to the field of traffic control systems using an inductive loop in a roadway to detect the presence of a vehicle by sensing interactions of the vehicle with the electro-magnetic field of the loop. More specifically, this invention relates to the field of devices and methods to activate such traffic control systems by passively simulating the presence of a vehicle.
2. Discussion of the Background
Control systems for traffic signals typically include a vehicle sensor having an inductive loop and utilizing the interaction between the electromagnetic field of the loop and a vehicle. The prior art is replete with descriptions of such systems as represented by U.S. Pat. No. 4,430,636 to Bruce, U.S. Pat. No. 4,472,706 to Hodge et al., U.S. Pat. No. 4,566,008 to Powers et al., U.S. Pat. No. 4,529,982 to Karlstrom et al., and U.S. Pat. No. 3,693,144 to Freidman.
The inductive loop vehicle sensors used in roadways as described in the aforementioned prior art typically include means for creating an electro-magnetic field, usually an inductive loop in the roadway (either embedded in the roadway or attached to the surface of the roadway). The inductive loop produces the electro-magnetic field, and such sensors further include means for utilizing changes in the electro-magnetic field to activate the control system. For example, when a vehicle such as a conventional automobile enters the electro-magnetic field, the inductance of the loop changes, and the sensor utilizes these changes in the inductance to sense the presence of the automobile and to activate the system.
However, many modern automobile designs utilize composite materials to the extent that such automobiles lack sufficient metallic presence to be detected by such sensors. Metallic presence as used herein refers to the combination of metallic mass and surface area necessary to activate the sensor. Also, some large vehicles, such as trucks, sit too high above the roadway to sufficiently interact with the field. Many other vehicles lacking sufficient metallic presence, such as motorcycles, bicycles, and wheelchairs, also fail to activate such control systems. (For convenience and clarity, all such automobiles, trucks, and other vehicles are hereinafter referred to as "undetectable vehicles" because such vehicles cannot be detected by such sensors, either due to their lack of sufficient metallic presence or high ground clearance, or both. In contrast, those vehicles having sufficient metallic presence and appropriate ground clearance so as to be detected by such sensors are hereinafter referred to as "detectable vehicles".) Furthermore, pedestrians obviously cannot activate such sensors.
One attempted solution to this problem is disclosed in U.S. Pat. No. 5,057,831 to Strang et al. This approach utilizes an active electronic circuit to derive a signal from the electromagnetic field of the sensing loop, amplify it, and impress upon such field a re-radiated signal in a phase creating within the field a disturbance similar to that which would occur upon the presence of a detectable vehicle. However, the Strang simulator requires a large number of components, packaging for these components, as well as a power source, all adding to the complexity and manufacturing costs of this simulator.
Hence, the need exists for an activation device that is lightweight, highly portable, inexpensive, and easy to use, as well as a method to ensure detection by such sensors of otherwise undetectable vehicles and pedestrians to allow safe and unobstructed traffic flow.